Speaking at an emotional hearing on federal gun control proposals, Baltimore County Police Chief James W. Johnson urged lawmakers Wednesday to close loopholes and ban assault weapon sales or risk more tragedies like the recent elementary school shooting in Connecticut.

The hearing before the Senate Judiciary Committee marked the first formal congressional debate on federal gun control since the Dec. 14 massacre in Newtown left 20 children and six adults dead. President Barack Obama and some Democratic lawmakers have called for re-establishing the 1994 federal ban on assault weapons, prohibiting high-capacity magazines and requiring all gun buyers to pass a background check.

"I've seen an explosion of fire power since the assault weapons ban expired," Johnson told lawmakers as he argued in support of those ideas. "It is common to find many shell casings at crime scenes when you go out and investigate these days. Victims are being riddled with multiple gun shots."

Former Arizona congresswoman Gabby Giffords, who was critically injured in a 2011 mass shooting, opened the hearing with a halting plea to committee members to "be courageous." Minutes later Giffords' husband, astronaut Mark Kelly, broke the news to the committee of Wednesday's mass shooting in a Phoenix office complex that left one person dead and several more injured.

Several senators fought back tears as they spoke of the Connecticut shooting or recounted subsequent conversations they have had with the parents of slain children.

But skepticism over some provisions proposed by Obama and Sen. Dianne Feinstein, the California Democrat who is leading the charge on the assault weapons ban, underscored the difficulty a comprehensive gun bill will face in Congress. LaPierre and some Republicans suggested more federal laws wouldn't ultimately reduce violence.

"Law-abiding gun owners will not accept blame for the acts of violent or deranged criminals," LaPierre said. "Nor do we believe that government should dictate what we can lawfully own and use to protect our families."

Sen. Lindsey Graham, a South Carolina Republican, argued there are situations that justify citizens owning a 15-round magazine, such as fending off multiple home intruders. Graham also called for increased security at schools.

"I think the way to interrupt the shooter if they come to a school house…is to have somebody like you, Chief Johnson, to meet them when hey come into the door," Graham said.

Wednesday's hearing was the latest in a series of high-profile appearances that have elevated Johnson's national profile as a spokesman on gun control. As the chairman of the National Law Enforcement Partnership to Prevent Gun Violence, Johnson also met with Obama at the White House in December.

The national platform has also brought criticism from gun rights advocates.

Johnson and Alabama Republican Sen. Jeff Sessions engaged in a testy exchange when the lawmaker asked the chief if he is concerned that the number of gun prosecutions in federal court had fallen significantly since 2006.

"No," Johnson responded.

"It doesn't concern you?" Sessions asked incredulously.

"Sir, you're not including local prosecutions," Johnson said. "I can't stand before you today and tell you of a single case in Baltimore County of an illegally possessed gun that was not prosecuted."

Sessions cut him off, raising his voice.

"Well, chief, are we trying to pass a federal law today or a state law?" he asked. "That's what you're calling for — is a federal law. We'd like to see the federal laws that are on the books enforced."

Johnson's testimony was also criticized by conservative websites, including some who confused Baltimore city with Baltimore County and, citing the city's crime, questioned the chief's standing as a spokesman on the issue.

Baltimore had 217 homicides last year. Baltimore County had 23.

Advocates said Johnson made a compelling case for gun control and held his own against LaPierre, a political veteran far more experienced at giving congressional testimony. Ladd Everitt, a spokesman for the Washington-based Coalition to Stop Gun Violence, said Johnson did "a very effective job."

At one point Sen. Dick Durbin, an Illinois Democrat, turned to Johnson and suggested the NRA believes the 2nd Amendment is included in the Constitution to give citizens "the firepower to fight back against you — against our government. So how do you conduct your business in enforcing the law not knowing what is behind that door?"

LaPierre could be heard trying to interject.

"I find it to be scary, creepy and simply just not based on logic," Johnson said.

Johnson was named police chief by then-County Executive Jim Smith in 2007 and became chair of the National Law Enforcement Partnership to Prevent Gun violence last September. The partnership is a coalition of national police leadership groups.

In that role, Johnson has called for extending background checks to all firearm purchases, limiting high-capacity magazines to 10 rounds, banning new semi-automatic assault weapons, and improving the National Instant Criminal Background Check System.

After the Sandy Hook massacre, Johnson appeared with County Executive Kevin Kamenetz at a press conference in Towson to call on state and federal legislators to strengthen gun control laws. Gov. Martin O'Malley renewed calls for tougher state gun laws in his State of the State address on Wednesday.

The county grappled with several gun incidents in schools last year, including a shooting on the first day of school at Perry Hall High in August. In October, the county launched a program to distribute free gun locks to residents, funded by the Baltimore County Police Foundation.

On Tuesday, county officials unveiled a $3.7 million plan that includes the expanded use of cameras in schools and the installation of upgraded electronic entry systems and a new visitor ID system.

But political momentum for action on gun control at the federal level appears to be slipping — partly because of concerns raised by Democratic senators from conservative states. One of the few provisions that does appear to have bipartisan support is stronger background checks of gun buyers.

Licensed firearms dealers must run those checks under federal law, but the requirement doesn't apply to guns bought at gun shows or from private sellers.

Johnson expressed skepticism about another idea some have floated to combat school violence: Arming teachers. He noted the precautions police use to maintain control of their guns and questioned how that would work in a classroom.

"How are you going to safeguard that weapon in a classroom of 16-year-old boys who want to touch it?" he asked.

Baltimore County Executive Kevin Kamenetz and Police Chief Jim Johnson on Monday called on state and federal lawmakers to strengthen laws on background checks for gun buyers and stop the sale of assault weapons and high-capacity magazines in the wake of the Connecticut school mass shooting that...

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